This invention relates generally to barrier coatings, and more particularly to barrier coatings for use in high temperature, aqueous environments.
Ceramic materials containing silicon and alloys containing silicon have been proposed for structures used in high temperature applications, such as gas turbine engines, heat exchangers, internal combustion engines, and the like. These materials are particularly useful in gas turbine engines which operate at high temperatures in aqueous environments.
The desired lifetime for components in such turbine applications may be tens of thousands of hours at temperatures above 1000° C., for example. However, the components are known to experience significant surface recession under exposure to high-temperature, aqueous environments. Volatile silicon-based gaseous species form at temperatures over about 1000° C. which causes the surface of the components to recede. The rate of recession may be 0.254 mm (0.010 in) or greater per 1000 hours, for example, depending on combustion conditions such as temperature and water vapor concentration in the combustion gas. This rate is unacceptably high for many component lifetime requirements.
One proposed solution is a three layer environmental barrier coating as described in U.S. Pat. Nos. 6,387,456 and 6,410,148, which includes a silicon bond coat, a mullite and barium strontium aluminosilicate (BSAS) intermediate coat, and a pure BSAS top coat. However, after exposure to temperatures above about 1200° C. for long periods of time, the recession rate can be unacceptably high for extended component lifetime requirements. Furthermore, chemical reactions occur which result in consumption of the bond coat and the intermediate coat, which may further reduce the component lifetime below the desired level.
During service in high temperature environments, the silicon bond coat oxidizes, creating an interfacial layer of silica at the bond coat-intermediate coat interface. A solid-state subsurface reaction between this silica layer and the intermediate coat occurs. As the reaction between the silica layer and the intermediate coat proceeds, the oxidation of the silicon bond coat increases, eventually consuming the bond coat and the intermediate coat. The interfacial layer grows from the production of reaction products while the bond coat and intermediate coat recede. The reaction products of the silica and intermediate coat are typically unstable and promote poor adherence between the bond coat and the intermediate coat. The reaction products may be present in multiple phases and may possess other undesirable properties such as high thermal expansion mismatch with the bond coat and the intermediate coat.
Once the intermediate coat is consumed, further chemical reactions may occur between the top coat and any remaining bond coat, leading to further consumption of the bond coat. Once the bond coat is consumed, the substrate is oxidized, leading to gas bubble formation and often spatling of the entire environmental barrier coating. Absent the protection of the environmental barrier coating, the substrate is exposed, resulting in a high rate of recession of the substrate and curtailing component life.
These and other drawbacks are present in known systems and techniques.